The present invention relates generally to power management systems for battery-powered devices and in particular to a power management system for an implantable devices.
Implantable devices require compact power sources and energy efficient electronics for prolonged device operation. Battery-powered devices must be explanted each time the power supply fails or reaches end of life (xe2x80x9cEOLxe2x80x9d). Ideally, the power management system of a battery-powered implantable device optimizes battery utilization by controlling battery consumption and by providing an elective replacement indicator (xe2x80x9cERIxe2x80x9d) which provides sufficient notice to the patient""s doctor that end of life is near without prematurely declaring ERI.
Several methods have been suggested for an ERI, however, battery characteristics vary and an accurate ERI is needed in the art. Generally speaking, internal battery resistance increases as the battery is used, however, transient internal resistances have been observed which vary in a complex function of battery life and history of current draw. The internal battery resistance will be called the xe2x80x9csteady state internal resistancexe2x80x9d throughout this document to distinguish it from the transient internal resistance. As stated before, the steady state internal resistance does vary with current consumption, and is therefore a function of current drawn from the cell, but it changes due to mechanisms which differ from the transient internal resistance mechanisms.
Furthermore, none of the previous power management systems have addressed a specific problem found in lithium-silver-vanadium-pentoxide batteries which arises when current is extracted from the battery in certain portions of the battery life curve. A lithium-silver-vanadium-pentoxide battery exhibits an abrupt increase in internal resistance in certain periods of battery life due to formation of a xe2x80x9cpassivation layerxe2x80x9d on the lithium surface following periods of relatively low current draw from the battery. The passivation layer creates a transient resistance which diminishes when current is drawn from the battery. This effect is called xe2x80x9cvoltage delayxe2x80x9d since the output voltage of the battery drops significantly upon current demand due to a large transient internal battery resistance. As current is extracted from the battery, the transient internal resistance is diminished and the output voltage of the battery returns to the ordinary output voltage for that portion of the battery lifetime.
The voltage delay effect may lower the battery output voltage below the reset voltage of the device electronics during high initial current draw. Even if the output voltage of the power source does not initially drop below the reset voltage the pulse delivery circuit may never draw enough energy to completely charge the high voltage output circuit due to a large steady state internal resistance of the battery.
Another problem with the previous power systems in implantable devices is that as the battery (or batteries, for multiple battery devices) approaches its EOL, the battery has increasing difficulty in providing adequate charge to the output capacitor to deliver a therapy pulse. Therefore, explantation and replacement of the device is performed earlier than necessary. The battery erroneously appears to have reached EOL because as current is drawn the voltage delay provides sufficient transient internal resistance to reduce the terminal voltage of the battery so as to signal elective replacement of the battery.
Therefore, there is a need in the art for an implantable power management system which extracts current from the power supply without requiring premature device replacement and without risking a reset of device hardware. The power system should also manage steady state internal battery resistance to ensure that the power supply can deliver a complete therapy pulse or, alternatively, signal the patient that the power supply is unsafe for charging the output circuits. There is yet further a need in the art for a power system which manages the transient internal battery resistance synonymous with the voltage delay effect so as to maximize battery life and reduce the number of device replacements.
The present invention includes several embodiments which provide a method and apparatus for a power management system for an implantable device, however, the teachings of the present disclosure apply equally to any device having a power supply with a variable internal resistance. For purposes of illustration, the present invention is described in the application of a power management system for an implantable cardioverter/defibrillator (ICD), however, the present invention is applicable to any battery powered device.
Most high voltage pulse delivery devices incorporate a power inverter to multiply the output voltage of the power supply. In devices having a flyback transformer energy is transferred to the output stage by periodically switching the output voltage of the power supply across the primary winding of the flyback transformer. The charging function requires substantially larger currents from the power supply than required by monitoring functions and the resulting voltage drop across the internal resistance of the power supply may result in the power supply output voltage falling below the reset voltage of the device circuits. Operation of the control circuits becomes unpredictable and unreliable when the supply output voltage approaches their reset voltage.
When the power supply is a battery, the internal battery resistance is a complex function of the battery physics and includes steady state internal resistance and a transient internal resistance. The steady state resistance is the increasing internal resistance of a battery over usage of the battery, absent the voltage delay effect. The transient resistance is the resistance caused by the passivation layer and is the source of the voltage delay effect.
In one embodiment, the power management system in an ICD uses a comparator to signal when the supply voltage drops below a predetermined threshold. The ICD control electronics pause charging until the power supply voltage exceeds a predetermined threshold. In this embodiment, electronics are used to record the time intervals when the charging activity was halted. In one embodiment, the amount of pause time per each charge cycle is accumulated. In another embodiment, the total charge time for each charge cycle is accumulated. The accumulated data may be used in an ERI system and for accurate determination of ERT.
In another embodiment the power management system incorporates a latch to determine whether the power supply voltage fell below the threshold while the microprocessor was performing other duties.
In yet another embodiment, a specialized control circuit is employed to sense when the supply voltage fell below the predetermined threshold. The specialized control circuit is designed for rapid recognition of the transition of the output voltage below the threshold This allows for pulse-by-pulse control of the charging activity.